(1) Field of the Invention
This invention relates to an image forming apparatus equipped with a photoconductive body which is gradually worn away as an image forming procedure is repeated many times (for example, a photoconductive drum having a surface formed of an organic photoconductive layer), especially to an image forming apparatus for compensating the sensitivity of the photoconductive body which would be deteriorated as the photoconductive body is worn away.
(2) Description of the Related Art
In a conventional copier equipped with a photoconductive drum, a surface thereof formed of an organic photoconductive layer is gradually worn away by a friction when a cleaning blade scratches off the residual toner on the surface after an image is transferred onto a copying paper.
Such a phenomenon deteriorates the sensitivity of the photoconductive drum for the following reason.
A surface potential V.sub.0 of the drum applied by a main charger and a thickness d of the organic photoconductive layer of the drum have the following relationship: ##EQU1## where Q: charge amount applied to the photoconductive drum per a unit area
C: capacitance per a unit area of the organic photoconductive layer PA0 .epsilon..sub.0 : dielectric constant in vacuum PA0 .epsilon..sub.r : relative dielectric constant in the organic photoconductive layer
As apparent from Equation (1), if the photoconductive drum is charged with the same surface potential V.sub.0 before and after the thickness d is reduced, the charge is accumulated in a larger amount in the latter case.
Accordingly, even if the photoconductive drum is exposed by the same light amount after the repetition of the image forming procedure as on the initial stage, the potential at the exposed portion is not lowered enough. In the normal development, such a phenomenon adheres an unnecessary toner on the exposed portion, as a result of which the copying paper gets fogging in a blank area. In the reverse development such as in a laser copier, the image density is lowered. In other words, the sensitivity of the photoconductive drum is lowered.
When the photoconductive drum is worn away much more and completes a life thereof, black streams appear on the copying paper or half-tone images are blurred. Since the life expectancy cannot be determined accurately in a conventional copier, the drum is renewed when the drum is still in a good condition or after the above problems occur.
Japanese Patent Publication No. 61-29505 has disclosed a copier for compensating the sensitivity of the photoconductive drum. The number of copies, the paper size and the exposure time are detected, and the copying conditions such as the light amount are adjusted in accordance with the predetermined relationship between each detected value and the characteristics of the photoconductive layer of the drum. In this construction, wherein a change in the thickness of the photoconductive layer is not directly detected, the compensation precision is not high.
According to U.S. Pat. No. 3,961,193, an influx current I.sub.pc, which flows to the photoconductive layer from the back side thereof and has the same amount as a charging current from the main charger to the surface of the photoconductive layer, is measured, and the output of the main charger is adjusted by comparing the measured I.sub.pc and the predetermined reference value. In such a construction, the surface potential V.sub.0 of the photoconductive layer can be kept at a certain level as long as the thickness of the photoconductive layer is kept the same. However, the reduction in the thickness d accompanies the decline in the surface potential V.sub.0. As a result, the image density is not high enough in the normal development while the copying paper gets fogging in the reverse development.